Paul A. Williams

X‐ray observation of electroclinic layer constriction and rearrangement in a chiral smectic‐A liquid crystal

An x‐ray scattering study of electroclinic layer constriction verifies the interpretation of the electroclinic effect as field‐induced molecular tilt. The tilt angles deduced from the layer spacing changes are in close agreement with those from optical measurements. Layer buckling, a consequence of the layer constriction, is also observed and may be the cause of the loss of optical contrast observed in electroclinic devices.

Vector Signal Characterization of High-Speed Optical Components by Use of Linear Optical Sampling With Milliradian Resolution

We demonstrate linear optical sampling measurements optimized for characterization of the signals produced by optical components. By sampling the optical electric field before and after the component, we isolate the full vector field (phase and amplitude) of the signal separate from the input laser drift. Synchronization of the low-jitter mode-locked sampling laser (e.g., frequency comb) with the modulation rate allows measurement of the phase with milliradian noise. As a demonstration, we measure 10-Gb/s differential phase-shift keying modulated data with several different lasers. The technique is readily scalable to systems of much higher bandwidth.

Achieving high absolute accuracy for Group-delay measurements using the Modulation phase-shift technique

We have developed a modulation phase-shift (MPS) system for measuring relative group delay (RGD) in optical components with high absolute accuracy and simultaneously high temporal and wavelength resolution. Our 200-MHz system has a 3.2-pm wavelength resolution and has demonstrated a group-delay resolution of 0.072 ps for repeated measurements of an artifact based on a hydrogen-cyanide gas cell. The expanded uncertainty (2/spl sigma/) is /spl plusmn/0.46 ps for a single spectral measurement (/spl sim/ 3.4-pm steps) of a narrow 20-ps group-delay feature of the artifact. To our knowledge, this is the first time that the sources of measurement uncertainty for this technique have been described and quantified. A method for predicting the group delay of the gas-cell artifact from measured absorption spectra is described, and an uncertainty analysis for the prediction method is also presented. The implementation required to achieve results of the highest accuracy for both measurements and predictions is discussed.

Studies of the higher order smectic phase of the large electroclinic effect material W317

Abstract We present studies of the large electroclinic effect material W317. We found via X-ray scattering, calorimetry and optical observation that quenching from the smectic A* phase results in several higher order phases including an orthogonal (hexatic) smectic with short range in-layer translational order and no interlayer order. We characterize the electroclinic response in the quenched phase, and determine its magnitude and response time as a function of electric field amplitude and temperature.

Chirp Characterization of External Modulators With Finite Extinction Ratio Using Linear Optical Sampling

We demonstrate a network monitoring technique for the frequency chirping of external modulators based on linear optical sampling. We present ¿-parameter waveforms of digital data modulation from simultaneously measured amplitude and phase. Digital data modulation was compared to sinusoidal modulation to demonstrate the measurement capabilities. The high sensitivity of our technique was used to resolve the detailed impact of a 23-dB extinction ratio on the chirp of a Mach-Zehnder modulator. Unlike some previous methods, a monochromator or optical fiber with known dispersion are not required.

Relative group delay measurements with 0.3 ps resolution: toward 40 Gbit/s component metrology

We present experimental and theoretical results for the measurement of RGD of molecular gas absorption lines, which have the potential to be stable and well-characterized standards. We show the experimental apparatus we constructed for measuring group delay. We used an extended-cavity diode laser having a 80 nm tuning range centered at 1560 nm, with a single mode linewidth of 5 MHz in a one-second time interval.

Chromatic dispersion measurement error caused by source amplified spontaneous emission

We demonstrate the degrading effect of noise from amplified spontaneous emission on measurements of chromatic dispersion using the modulation phase-shift method. Dramatic performance improvement is achieved with a narrow-band tracking filter.

Direct Measurement of Vector Polarization-Mode Dispersion From Repeated Random Data by Use of Linear Optical Sampling

Polarization-dependent optical sampling techniques measure the complex electric field, allowing one to monitor the degrading effects of a communication channel. Here we show the ability to extract the polarization-mode dispersion of the transmission channel from a remote measurement of transmitted repeated 10-Gb/s differential phase-shift keying modulated data. This approach is independent of modulation format and data rate. We demonstrate results both with and without referencing to the laser phase. We achieve a differential group delay resolution of 0.2 ps for an unreferenced measurement.

Optical Metrology for Wavelength-Division-Mulitplexed Fiber Communications

In response to rapid changes in fiber-optic technology, the Optoelectronics Division of the National Institute of Standards and Technology has been developing techniques and standards to support the measurement of optical components and subsystems used in wavelength-division-multiplexed optical fiber communication systems. Current projects include the development of wavelength calibration transfer standards and the accurate measurement of spectral response, dispersion, and polarization dependence of optical fiber and components.

Coherent Measurements with Fiber-Laser Frequency Combs

The coherent and broad spectral output of fiber-laser frequency combs can be exploited for a variety of high-resolution measurements outside of conventional frequency metrology. We will discuss recent measurements in spectroscopy, ranging, and telecommunication components.